The molten carbonate fuel cell is an electrochemical device producing DC electricity by the oxidation of a fuel gas at an invariant electronically-conducting anode, with simultaneous reduction of oxygen, typically from air, at an invariant electronically-conducting cathode. The anodes and cathodes are separated by a thin layer of electrolyte, which includes a mixture of molten alkali metal carbonates in the temperature range of operation, typically 600.degree. C. to 750.degree. C. The carbonate mixture is maintained in position by capillary action with the pores of a fine stable non-electrically-conducting powder, for example, lithium aluminate, which also serves to prevent electronic shorts between the electrodes from occurring. Typically, the invariant anode is a porous sintered nickel or alloy material of sufficient thickness to store a required inventory of electrolyte, to make up losses by evaporation during a lifetime of the cell. Also, the sinter porosity is matched to the particular electrolyte binder, so that the binder remains filled by capillarity and the anode maintains an open interface with the gas phase in the pores. The electrolyte-solid electrode-gas three phase boundary is selected to provide a maximum possible area for reaction to take place, so that the electrical voltage losses resulting from irreversible polarizations at local reaction rates are minimized. The cathode typically consists of a porous invariant lithium doped nickel oxide material of appropriate porosity characteristics. The cathode is normally a thin structure due to its relatively high electronic resistance. To avoid flooding of the structure by capillary action, nickel oxide or other cathodic material of the desired characteristics is used. The anode and cathode material are maintained in their invariant conditions by the reducing and oxidizing natures of the gases supplied to them.
The remainder of the individual unit cell structure consists of anodic and cathodic current collectors, which are directly applied to the respective electrodes, and which have the requisite invariance, electronic conductivity, and ability for allowing gases to come in contact with the electrodes, typically channeled or porous structures, or both, with gases fed through the structure across the faces of the electrodes. The current collectors contact separator plates (bipolar plates) which are common to the anode and cathodes of two adjacent cells, so that a stack of individual cells similar to a Volta pile is thus formed. Each stack is individually equipped with a manifold to supply and exit gas, and with a gas-type corrosion-resistant, electronically-insulating sealer on the edge, to prevent both mixing of gases and electronic shorting of cells. The individual cell is thus sealed in contact with a frame-like structure, which typically is formed of a metal with anti-corrosion protection, such as aluminized stainless steel. The electrolyte composite layer seals across this space to the metal frame. During normal operation of a commercial cell, excursions of gas pressure can occur when operation takes place under high pressure conditions to increase performance. Therefore, the structure must be able to resist these pressure excursions without failure or cross-leakage. Furthermore, shut-downs of cell stacks occur from time to time. This requires the cell components to be thermally cycled several times, down to ambient temperature. The most sensitive component of the cell stack is the electrolyte composite layer, with a coefficient of expansion which differs from that of metal components since it is susceptible to cracking under these conditions, thus causing catastrophic failure on start-up.
At the present time, most molten carbonate fuel cells are constructed using dense hot-pressed electrolyte tiles containing electrolyte and binder, the components being premixed in powder form. High pressures are necessary to reach the densities required to prevent gas crossover. For large-scale production, such tiles cannot be made in practical sizes, e.g., several square feet, with the existing press technology. Also, the tile itself, which is a viscous fluid at cell operating temperatures, is a structural component and must be capable of free-standing, at least during manufacture. Thus, it must be relatively thick in large sizes, so that the internal cell IR losses would be high. The thick layers resulting are not thermally cyclable, and require structural reinforcement of some type (e.g., invariant metal wires or other components) if tendency to cracking is to be reduced or eliminated. Even so, this approach is not fully successful.
Among the problems inherent in the electrolyte layer of a molten carbonate fuel cell are (1) the ability to manufacture the elctrolyte layer, (2) the ability to thermally cycle the electrolyte layer, (3) maintenance of gas integrity through the electrolyte layer under different pressure conditions, and (4) the minimization of the thickness of the electrolyte layer to therefore minimize the IR drop and maximize cell performance.